The present invention relates to an image formation apparatus for forming an image by printing dots while feeding a recording medium relatively in a direction virtually perpendicular to a linear arrangement direction of recording elements of a recording head.
As an example of the image formation apparatus of this type, conventionally known is an ink-jet type line printer. Such an ink-jet type line printer has a line recording head consisting of a plurality of ink chambers arranged in a main scanning direction (i.e., a line direction) and ink jet-out ports corresponding to the ink chambers. To improve image quality, it is necessary to form the ink jet-out ports in the same size and shape and arrange them at the same intervals (pitches). However, in practice, it is difficult to form all the ink jet-out ports such that they are identical. If the ink jet-out ports formed in a head manufacturing step slightly differ in size and shape, the difference in size and shape has an effect on amounts and directions of ink jetting out from the ink jet-out ports. The resultant image is non-uniform in concentration and thus degraded in quality .
As shown, for example, in FIG. 9, in the case where ink is jetted out from individual ink jet-out ports 2 of the line recording head 1 in the same amount and in the same direction, ink dots 3 jetted out from the ink jet-out ports 2 will be identical. Consequently, pixel dots 5 will be printed in identical size and direction on a recording paper (recording medium) 4. As a whole, the formed image is uniform in concentration.
In practice, however, the ink jet-out ports 2 of the line recording head 1 are formed irregularly in size and shape, as shown in FIG. 10. The ink dots 6 jetted out from the ink-jet ports 2 therefore differ in jet out amount and direction. As a result, pixel dots 7 printed on the recording paper 4 differ in size and direction. If such dots are overlapped excessively, a black stripe is formed, whereas if the dots are printed at much larger intervals than required, a white stripe is formed. The resultant image is non-uniform in concentration.
To suppress image formation from being non-uniform in concentration as much as possible, known is a control method shown in FIG. 11. In this method, assuming that a line recording head 11 consists of, for example, eight ink-jet ports 12, the line recording head 11 is controlled by moving the line recording head 11 in an arrangement direction of the ink jet-out ports 12 (recording elements) while a recording paper 14 is fed repeatedly in a direction virtually perpendicular to the arrangement direction of the ink jet-out ports 12, multiple times. In this case, since ink dots 13 jetted out from ink jet-out ports 12 differ in jet-out amount and direction, an image is formed on the recording paper 14 (wound around a drum three times) by printing dots alternately up and down in a staggering manner while controlling movement of the line recording head 11 at every round.
In other words, at a first round of the drum, first to fourth ink jet-out ports 12.sub.5 to 12.sub.8 from the right hand side of the line recording head 11 are used. More specifically, the ink jet-out ports 12.sub.5 and 12.sub.7 are used to print dots of odd lines, whereas the ink jet-out ports 12.sub.6 and 12.sub.8 are used to print dots of even lines. At a second round of the drum, the line recording head 11 is slid rightward by four pitches of the ink jet-out ports. At this time, all ink jet-out ports 12.sub.1 to 12.sub.8 of the line recording head 11 are used. More specifically, dots of odd lines are printed by the ink jet-out ports 12.sub.2, 12.sub.4, 12.sub.6, 12.sub.8, whereas dots of even lines are printed by the ink jet-out ports 12.sub.1, 12.sub.3, 12.sub.5, 12.sub.7. At a final round (third round), the line recording head 11 is slid further rightward by four pitches of ink jet-out ports. In this case, first to fourth ink jet-out ports 12.sub.1 -12.sub.4 from the lefthand side are used. That is, the ink jet-out ports 12.sub.1 and 12.sub.3 are used for printing dots of odd lines, whereas the ink jet out-ports 12.sub.2 and 12.sub.4 are used to print dots of even lines.
In the recording method of this type, if the ink jet-out ports 12.sub.1 -12.sub.8 are irregularly formed in size and shape, the effect of the irregularity in size and shape on the image formed on the recording paper 14 is reduced in half. As a result, the black and white stripes as shown in FIG. 10 are not clearly visible, so that image formation that is non-uniform in concentration can be suppressed.
Furthermore, another control method is known which is concerned with increasing resolution. In the control method, the line recording head 11 is slid in the arrangement direction of the ink jet-out ports 12 by L/K (L is an arrangement interval of the ink jet-out ports 12), while the recording paper 14 is repeatedly fed in the direction virtually perpendicular to the arrangement direction of the ink jet-out ports 12, multiple times. In the case where the recording paper 14 is wound around the drum twice, pixel dots 15 (indicated by hatched lines declining rightward in the figure) are printed on the recording paper 14 at a first round of the drum, as shown in FIG. 12. At the second round, the line recording head 11 is slid rightward by L/2 (L is an arrangement interval of the ink jet-out ports 12), and then, recording pixel dots 16 (indicated by hatched lines declining leftward in the figure) are printed on the recording paper 14. In this manner, the resolution of the formed image is improved twice as high as that of the line recording head 11.
As described, in the image formation method for improving the resolution of the image by controlling the movement of the line recording head 11, if the ink dots jetted out from the ink jet-out ports 12 of the line recording head 11 differ in jet-out amount and direction, the black stripe (which is formed by excessively overlapping dots than required) and the white stripe (which is formed by printing dots at intervals which are excessively far from each other than required) emerge, as shown in FIG. 13. As a result, the obtained image is non-uniform in concentration as a whole.
To overcome this problem, it is considered that the image is formed by two control methods in combination, as shown in FIG. 14. One is a control method in which the line recording head 11 is slid by L/2 (L is an arrangement interval of the ink jet-out ports 12) and the other is a control method in which dots are printed on the recording paper 14 alternately up and down in a staggering manner. To be more specific, at a first round of the drum, pixel dots 17 (indicated by a lattice pattern in the figure) are printed on the recording paper 14. At a second round of the drum, the line recording head 11 is slid rightward by L/2, and then, pixel dots 18 (indicated by hatched line declining rightward) are printed on the recording paper 14. At a third round of the drum, the line recording head 11 is slid further rightward by L/2, and then, pixel dots 19 (indicated by hatched lines declining leftward) are printed. At a fourth round of the drum, the line recording head 11 is further slid rightward by L/2 and then pixel dots 20 (indicated by a white circle in the figure) are printed on the recording paper 14. In this manner, the resolution of the image is improved twice as high as that of the line recording head 11. At the same time, image formation that is non-uniform in concentration due to irregular arrangement of the ink jet-out ports can be successively suppressed.
However, the aforementioned control technique has a problem in that it takes too much time to form the image since the time required for image formation is doubled as compared to the control method which only increases the resolution.